That subsidence, easy to miss and easier still to dismiss as ordinary agricultural disturbance, is the only visible sign of a souterrain lying beneath the surface, its roof long since unable to hold.

Souterrains are underground stone-lined passages or chambers, typically built during the early medieval period in Ireland, roughly between the seventh and twelfth centuries. They were constructed beneath or adjacent to ringforts, the circular enclosed farmsteads that were the most common form of rural settlement at the time, and their precise purpose has been debated for generations. Refuge, cold storage, and ventilation have all been proposed, and the answer may well vary from site to site. At Kilnagnady, the association with the adjacent ringfort follows a pattern found across the country, where souterrains and their parent enclosures were built and used as part of the same domestic complex. The collapsed area in the north-eastern quadrant is the kind of surface evidence archaeologists rely on when no excavation has taken place; the ground above a dry-stone-roofed passage tends to sink gradually as the capstones shift or fracture, leaving a tell-tale depression that marks the underground outline below.